1. Field of the Invention
The invention relates to a vehicle power transmission apparatus provided with a torque converter having a lockup clutch.
2. Description of Related Art
In recent years, automatic transmission torque converters having a lockup clutch for directly connecting the input and output sides of the torque converter have been used.
For example, Japanese Patent Application Publication No. 2008-51316 (JP-A-2008-51316) (refer to paragraphs 0061 to 0067) describes a system that controls, using a hydraulic pressure control circuit, the differential pressure between the apply side hydraulic chamber and the release side hydraulic chamber of a torque converter (lockup differential pressure) to apply a lockup clutch (lockup ON) and release it (lockup OFF).
More specifically, the hydraulic pressure control circuit described above controls the operation hydraulic pressures input, respectively, to the apply side hydraulic chamber and release side hydraulic chamber of the torque converter by turning on and off a spool valve, called “lockup control valve”, using a duty solenoid valve for lockup control (will be referred to as “lockup control duty solenoid valve”). Normally, the lockup clutch is applied (lockup ON) in response to the vehicle speed reaching a predetermined lockup ON determination value after the start of the vehicle.
If an abnormality causing the solenoid of the lockup control duty solenoid valve to remain in an on state (will hereinafter be referred to as “solenoid ON abnormality”) occurs, the lockup control duty solenoid valve continues to output a control hydraulic pressure to the lockup control valve, resulting in the lockup clutch stuck in the applied state. If the vehicle is stopped while such an abnormality is occurring, the engine will stall when the vehicle starts the next time.
In order to avoid such an engine stall at the start of the vehicle, fail-safe control is executed which forces the lockup clutch to be placed in the released state for the period from the start of the vehicle to the vehicle speed reaching a predetermined speed.
In the fail-safe control, a control hydraulic pressure for releasing the lockup clutch (lockup OFF) is input to the lockup control valve from another spool valve that is called “clutch apply control valve” and provided in the hydraulic pressure control circuit, thus forcibly releasing the lockup clutch independently of whether or not the control hydraulic pressure for applying the lockup clutch (lockup ON) is being input to the lockup control valve from the lockup control duty solenoid valve.
An in-vehicle abnormality diagnosis system (OBD: Onboard Diagnosis System) incorporated in an automatic transmission control unit performs diagnosis on a number of diagnosis items. For example, it has been proposed to use such a diagnosis system to detect whether the lockup control duty solenoid valve is suffering from a solenoid ON abnormality and record the particulars of the detected abnormality in a memory.
To accomplish diagnosis on this diagnosis item, that is, diagnosis for a solenoid ON abnormality of the lockup control duty solenoid valve, the following diagnosis method is used. In a case where the vehicle has started with the fail-safe control executed, the fail-safe control is discontinued by a fail-safe control discontinuation command being output in response to the vehicle speed reaching a predetermined fail-safe OFF determination value, and then the amount of slip at the toque converter (will be referred to as “the slip amount of the torque converter”) is examined.
After the fail-safe control has been thus discontinued, if the lockup control duty solenoid valve is functional, the lockup clutch remains in the released state (lockup OFF), and therefore the slip amount of the torque converter gradually increases from the discontinuation of the fail-safe control, and if the lockup control duty solenoid valve is suffering from a solenoid ON abnormality, on the other hand, the lockup clutch is applied (lockup ON) and therefore the slip amount of the torque converter decreases and then becomes zero.
As such, it is possible to determine that the lockup control duty solenoid valve is suffering from a solenoid ON abnormality when it is detected that the slip amount of the torque converter has become zero. However, in order to examine whether such a phenomenon is a temporary phenomenon, preferably, it is determined whether the slip amount of the torque converter has continuously been zero for a predetermined period of time. This determination process is necessary to improve the abnormality diagnosis accuracy.
In consideration of the nature, purpose, and the like, of the abnormality diagnosis that examines the slip amount of the torque converter in a situation as described above, the fail-safe OFF determination value needs to be set to a value smaller than the lockup ON determination value (i.e., a value corresponding to a lower vehicle speed). The smaller the difference between the fail-safe OFF determination value and the lockup ON determination value, the more likely the time allowed for executing processes for improving the abnormality diagnosis accuracy, that is, the duration of the abnormality diagnosis is to be insufficient. In view of this, the applicant has conceived of, as an option for securing sufficient time for the abnormality diagnosis, outputting a lockup ON command (i.e., a command for applying the lockup clutch) a predetermined time after the vehicle speed reaches the lockup ON determination value, rather than outputting the lockup ON command immediately when the vehicle speed reaches the lockup ON determination value, as mentioned above.
In addition, Japanese Patent Application Publications No. 2006-300285 (JP-A-2006-300285) and No. 2008-185088 (JP-A-2008-185088) each describe executing a preliminary abnormality determination process, in addition to an abnormality determination process, when detecting whether the control of a lockup mechanism is being properly executed.
Meanwhile, although the system described in JP-A-2008-51316 can determine whether the lockup control duty solenoid valve is suffering from a solenoid ON abnormality, the following should be taken into account.
In recent years, there have been demands for increasing the rate at which a precondition for performing, as one of diagnosis items for an in-vehicle abnormality diagnosis system (OBD), diagnosis for determining whether a lockup control duty solenoid valve is suffering from a solenoid ON abnormality (execution allowance condition) is satisfied when the vehicle has started from a situation where the lockup control duty solenoid valve is substantially functional (i.e., when the vehicle has started without the fail-safe control executed, such as when the vehicle has started after stopping without an engine stall). However, JP-A-2008-51316 describes neither technical concept nor specific structure for meeting the demands mentioned above, leaving a room for improvement.
Meanwhile, in order for the precondition for performing the solenoid ON abnormality diagnosis to be satisfied when the vehicle has started from a situation where the lockup control duty solenoid valve was functional, it is necessary to incorporate a measure or measures for improving the abnormality diagnosis accuracy, such as the one described above, into the precondition. In such a case (except cases where a solenoid ON abnormality occurs immediately after the vehicle has started), however, the output of the lockup ON command needs to be delayed despite the fact that the lockup control duty solenoid valve is functional, and therefore, for example, the time at which to start applying the lockup clutch becomes later than the target time, which may result in a reduction of the fuel economy. Thus, when the vehicle is started from a situation where the lockup control duty solenoid valve is functional, it is difficult to satisfy the precondition for performing the solenoid ON abnormality diagnosis and therefore it is difficult to increase the rate at which the precondition is satisfied.
JP-A-2006-300285 and JP-A-2008-185088 each describe no technical concept and no specific structure for meeting the aforementioned demands regarding in-vehicle abnormality diagnosis systems.